Why is 9i pausing?

I'm experiencing an intermittent performance problem reading and
writing a 9i database from Sqlplus. I'm running Sqlplus on my laptop
and the DB is on a Sun box in the same office. It seems that if my
connection to the DB is inactive for a period of time, there is usually
a really bad lag when next I try to read or write the DB. If I do a few
things in quick succession, they all seem to execute reasonably
quickly. Whether a period of inactivity forces Oracle to go through
some sort of extended handshaking when activity picks up again, I don't
know, but that's what it feels like to me.

In my read tests, the data - once it starts to stream - comes back at a
decent rate, but there's sometimes a delay of upto a minute before it
starts to send. I got a similar result with write tests: I created 1000
rows in next to no time but when I tried it a second time, there was a
pause of about a minute before it started, then it created the 1000
rows pretty quickly.

FWIW the Sun box has 1.5GB memory, is serving a total of three 9i
databases, and is inactive apart from the tests I'm doing. My database
is running on default instance parameters (it was created as a "new
database" in DBCA, and I just accepted all the defaults).